1. Field of the Invention
The present invention relates generally to the field of modem circuits, and more particularly, to a telephone line interface circuit incorporating low-voltage transistors to suppress high-voltage, short-term voltage surges.
2. Description of Related Art
FIG. 1 illustrates a typical prior-art telephone line interface circuit 10 for use in a modem circuit. Before the modem accesses the telephone line, the telephone line interface circuit 10 is normally in an xe2x80x9con-hookxe2x80x9d state, meaning that the modem circuit is disconnected from the telephone line. In order for the modem to go xe2x80x9coff-hookxe2x80x9d (i.e. access the telephone line), an off-hook signal xe2x80x9cOHxe2x80x9d 103 is applied to a relay switch 102. The transistor 106 draws a loop current sufficient to notify the telephone system that the modem is now off-hook. When the circuit is on-hook, the relay 102 effectively isolates the transistor from the telephone line, and any high-voltage surges as well. Thus, the transistor 106 only needs to be protected from high-voltage surges when the modem is off-hook.
High-voltage surges on telephone lines may result from, for example, lightening strikes. These strikes can be simulated using test pulses of approximately 800 V for relatively short periods. Such high-voltage spikes or surges could easily damage unprotected equipment connected to a telephone line. Therefore, the FCC requires all products intended for use with the telephone system to pass a xe2x80x9cPart 68xe2x80x9d high-voltage pulse test.
As a first level of protection against high-voltage surges on the telephone line, a 400 V peak (175 V RMS) MOV (metal oxide varistor) 101 is placed across the tip and ring lines of the circuit of FIG. 1 to dissipate any voltage surges. However, a 400 V peak surge would still likely exceed the limits of the transistor 106. Therefore, a zenor diode 104 (typically 13 V) is also placed across the transistor 106 to provide an extra level of protection.
This voltage surge protection scheme works well for a telephone line interface circuit incorporating a relay 102. However, newer telephone line interface circuits have eliminated the relay 102 in order to reduce the size and cost of the interface circuitry. Merely placing a zenor diode 104 across the transistor 106 is no longer a valid option, since without a relay switch, the zenor diode 104 will constantly draw loop current and the modem could never go on-hook. Also, using a high-voltage transistor capable of withstanding a high-voltage surge is not practical, since high-voltage transistors do not provide adequate signal distortion performance. Also, for a given design, only a single vendor""s transistor could be incorporated to provide adequate voltage and distortion characteristics. Thus, there is a need for a high-voltage surge protection design for use with relay-less telephone line interface circuits.
The present invention is a telephone line interface circuit incorporating low-voltage transistors to suppress high-voltage, short-term voltage surges. Specifically, a low-voltage transistor such as a Central Semiconductor C2TA44, a Motorola MPSA42, or similar such transistors actually can withstand a high-voltage spike exceeding the manufacturers specified parameters. The present invention exploits this undocumented feature by employing such a transistor in combination with a metal oxide varistor in order to provide adequate voltage surge protection. Thus, a zenor diode is not necessary (avoiding on-hook problems) and the relay-less circuit still passes the FCC Part 68 test.